//////////////////////////////////////////////////////////////////////
////                                                              ////
////  uart_regs.v                                                 ////
////                                                              ////
////                                                              ////
////  This file is part of the "UART 16550 compatible" project    ////
////  http://www.opencores.org/cores/uart16550/                   ////
////                                                              ////
////  Documentation related to this project:                      ////
////  - http://www.opencores.org/cores/uart16550/                 ////
////                                                              ////
////  Projects compatibility:                                     ////
////  - WISHBONE                                                  ////
////  RS232 Protocol                                              ////
////  16550D uart (mostly supported)                              ////
////                                                              ////
////  Overview (main Features):                                   ////
////  Registers of the uart 16550 core                            ////
////                                                              ////
////  Known problems (limits):                                    ////
////  Inserts 1 wait state in all WISHBONE transfers              ////
////                                                              ////
////  To Do:                                                      ////
////  Nothing or verification.                                    ////
////                                                              ////
////  Author(s):                                                  ////
////      - gorban@opencores.org                                  ////
////      - Jacob Gorban                                          ////
////      - Igor Mohor (igorm@opencores.org)                      ////
////                                                              ////
////  Created:        2001/05/12                                  ////
////  Last Updated:   (See log for the revision history           ////
////                                                              ////
////                                                              ////
//////////////////////////////////////////////////////////////////////
////                                                              ////
//// Copyright (C) 2000, 2001 Authors                             ////
////                                                              ////
//// This source file may be used and distributed without         ////
//// restriction provided that this copyright statement is not    ////
//// removed from the file and that any derivative work contains  ////
//// the original copyright notice and the associated disclaimer. ////
////                                                              ////
//// This source file is free software; you can redistribute it   ////
//// and/or modify it under the terms of the GNU Lesser General   ////
//// Public License as published by the Free Software Foundation; ////
//// either version 2.1 of the License, or (at your option) any   ////
//// later version.                                               ////
////                                                              ////
//// This source is distributed in the hope that it will be       ////
//// useful, but WITHOUT ANY WARRANTY; without even the implied   ////
//// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR      ////
//// PURPOSE.  See the GNU Lesser General Public License for more ////
//// details.                                                     ////
////                                                              ////
//// You should have received a copy of the GNU Lesser General    ////
//// Public License along with this source; if not, download it   ////
//// from http://www.opencores.org/lgpl.shtml                     ////
////                                                              ////
//////////////////////////////////////////////////////////////////////
//
// CVS Revision History
//
// $Log: not supported by cvs2svn $
// Revision 1.41  2004/05/21 11:44:41  tadejm
// Added synchronizer flops for RX input.
//
// Revision 1.40  2003/06/11 16:37:47  gorban
// This fixes errors in some cases when data is being read and put to the FIFO at the same time. Patch is submitted by Scott Furman. Update is very recommended.
//
// Revision 1.39  2002/07/29 21:16:18  gorban
// The uart_defines.v file is included again in sources.
//
// Revision 1.38  2002/07/22 23:02:23  gorban
// Bug Fixes:
//  * Possible loss of sync and bad reception of stop bit on slow baud rates fixed.
//   Problem reported by Kenny.Tung.
//  * Bad (or lack of ) loopback handling fixed. Reported by Cherry Withers.
//
// Improvements:
//  * Made FIFO's as general inferrable memory where possible.
//  So on FPGA they should be inferred as RAM (Distributed RAM on Xilinx).
//  This saves about 1/3 of the Slice count and reduces P&R and synthesis times.
//
//  * Added optional baudrate output (baud_o).
//  This is identical to BAUDOUT* signal on 16550 chip.
//  It outputs 16xbit_clock_rate - the divided clock.
//  It's disabled by default. Define UART_HAS_BAUDRATE_OUTPUT to use.
//
// Revision 1.37  2001/12/27 13:24:09  mohor
// lsr[7] was not showing overrun errors.
//
// Revision 1.36  2001/12/20 13:25:46  mohor
// rx push changed to be only one cycle wide.
//
// Revision 1.35  2001/12/19 08:03:34  mohor
// Warnings cleared.
//
// Revision 1.34  2001/12/19 07:33:54  mohor
// Synplicity was having troubles with the comment.
//
// Revision 1.33  2001/12/17 10:14:43  mohor
// Things related to msr register changed. After THRE IRQ occurs, and one
// character is written to the transmit fifo, the detection of the THRE bit in the
// LSR is delayed for one character time.
//
// Revision 1.32  2001/12/14 13:19:24  mohor
// MSR register fixed.
//
// Revision 1.31  2001/12/14 10:06:58  mohor
// After reset modem status register MSR should be reset.
//
// Revision 1.30  2001/12/13 10:09:13  mohor
// thre irq should be cleared only when being source of interrupt.
//
// Revision 1.29  2001/12/12 09:05:46  mohor
// LSR status bit 0 was not cleared correctly in case of reseting the FCR (rx fifo).
//
// Revision 1.28  2001/12/10 19:52:41  gorban
// Scratch register added
//
// Revision 1.27  2001/12/06 14:51:04  gorban
// Bug in LSR[0] is fixed.
// All WISHBONE signals are now sampled, so another wait-state is introduced on all transfers.
//
// Revision 1.26  2001/12/03 21:44:29  gorban
// Updated specification documentation.
// Added full 32-bit data bus interface, now as default.
// Address is 5-bit wide in 32-bit data bus mode.
// Added wb_sel_i input to the core. It's used in the 32-bit mode.
// Added debug interface with two 32-bit read-only registers in 32-bit mode.
// Bits 5 and 6 of LSR are now only cleared on TX FIFO write.
// My small test bench is modified to work with 32-bit mode.
//
// Revision 1.25  2001/11/28 19:36:39  gorban
// Fixed: timeout and break didn't pay attention to current data format when counting time
//
// Revision 1.24  2001/11/26 21:38:54  gorban
// Lots of fixes:
// Break condition wasn't handled correctly at all.
// LSR bits could lose their values.
// LSR value after reset was wrong.
// Timing of THRE interrupt signal corrected.
// LSR bit 0 timing corrected.
//
// Revision 1.23  2001/11/12 21:57:29  gorban
// fixed more typo bugs
//
// Revision 1.22  2001/11/12 15:02:28  mohor
// lsr1r error fixed.
//
// Revision 1.21  2001/11/12 14:57:27  mohor
// ti_int_pnd error fixed.
//
// Revision 1.20  2001/11/12 14:50:27  mohor
// ti_int_d error fixed.
//
// Revision 1.19  2001/11/10 12:43:21  gorban
// Logic Synthesis bugs fixed. Some other minor changes
//
// Revision 1.18  2001/11/08 14:54:23  mohor
// Comments in Slovene language deleted, few small fixes for better work of
// old tools. IRQs need to be fix.
//
// Revision 1.17  2001/11/07 17:51:52  gorban
// Heavily rewritten interrupt and LSR subsystems.
// Many bugs hopefully squashed.
//
// Revision 1.16  2001/11/02 09:55:16  mohor
// no message
//
// Revision 1.15  2001/10/31 15:19:22  gorban
// Fixes to break and timeout conditions
//
// Revision 1.14  2001/10/29 17:00:46  gorban
// fixed parity sending and tx_fifo resets over- and underrun
//
// Revision 1.13  2001/10/20 09:58:40  gorban
// Small synopsis fixes
//
// Revision 1.12  2001/10/19 16:21:40  gorban
// Changes data_out to be synchronous again as it should have been.
//
// Revision 1.11  2001/10/18 20:35:45  gorban
// small fix
//
// Revision 1.10  2001/08/24 21:01:12  mohor
// Things connected to parity changed.
// Clock devider changed.
//
// Revision 1.9  2001/08/23 16:05:05  mohor
// Stop bit bug fixed.
// Parity bug fixed.
// WISHBONE read cycle bug fixed,
// OE indicator (Overrun Error) bug fixed.
// PE indicator (Parity Error) bug fixed.
// Register read bug fixed.
//
// Revision 1.10  2001/06/23 11:21:48  gorban
// DL made 16-bit long. Fixed transmission/reception bugs.
//
// Revision 1.9  2001/05/31 20:08:01  gorban
// FIFO changes and other corrections.
//
// Revision 1.8  2001/05/29 20:05:04  gorban
// Fixed some bugs and synthesis problems.
//
// Revision 1.7  2001/05/27 17:37:49  gorban
// Fixed many bugs. Updated spec. Changed FIFO files structure. See CHANGES.txt file.
//
// Revision 1.6  2001/05/21 19:12:02  gorban
// Corrected some Linter messages.
//
// Revision 1.5  2001/05/17 18:34:18  gorban
// First 'stable' release. Should be sythesizable now. Also added new header.
//
// Revision 1.0  2001-05-17 21:27:11+02  jacob
// Initial revision
//
//

// synopsys translate_off
`include "timescale.v"
// synopsys translate_on

`include "uart_defines.v"

`define UART_DL1 7:0
`define UART_DL2 15:8

module uart_regs (clk,
		  wb_rst_i, wb_addr_i, wb_dat_i, wb_dat_o, wb_we_i, wb_re_i, 

		  // additional signals
		  modem_inputs,
		  stx_pad_o, srx_pad_i,

`ifdef DATA_BUS_WIDTH_8
`else
		  // debug interface signals	enabled
		  ier, iir, fcr, mcr, lcr, msr, lsr, rf_count, tf_count, tstate, rstate,
`endif				
		  rts_pad_o, dtr_pad_o, int_o
`ifdef UART_HAS_BAUDRATE_OUTPUT
		  , baud_o
`endif

		  );

   input clk;
   input wb_rst_i;
   input [`UART_ADDR_WIDTH-1:0] wb_addr_i;
   input [7:0] 			wb_dat_i;
   output [7:0] 		wb_dat_o;
   input 			wb_we_i;
   input 			wb_re_i;

   output 			stx_pad_o;
   input 			srx_pad_i;

   input [3:0] 			modem_inputs;
   output 			rts_pad_o;
   output 			dtr_pad_o;
   output 			int_o;
`ifdef UART_HAS_BAUDRATE_OUTPUT
   output 			baud_o;
`endif

`ifdef DATA_BUS_WIDTH_8
`else
   // if 32-bit databus and debug interface are enabled
   output [3:0] 		ier;
   output [3:0] 		iir;
   output [1:0] 		fcr;  /// bits 7 and 6 of fcr. Other bits are ignored
   output [4:0] 		mcr;
   output [7:0] 		lcr;
   output [7:0] 		msr;
   output [7:0] 		lsr;
   output [`UART_FIFO_COUNTER_W-1:0] rf_count;
   output [`UART_FIFO_COUNTER_W-1:0] tf_count;
   output [2:0] 		     tstate;
   output [3:0] 		     rstate;

`endif

   wire [3:0] 			     modem_inputs;
   reg 				     enable;
`ifdef UART_HAS_BAUDRATE_OUTPUT
   assign baud_o = enable; // baud_o is actually the enable signal
`endif


   wire 			     stx_pad_o;		// received from transmitter module
   wire 			     srx_pad_i;
   wire 			     srx_pad;

   reg [7:0] 			     wb_dat_o;

   wire [`UART_ADDR_WIDTH-1:0] 	     wb_addr_i;
   wire [7:0] 			     wb_dat_i;


   reg [3:0] 			     ier;
   reg [3:0] 			     iir;
   reg [1:0] 			     fcr;  /// bits 7 and 6 of fcr. Other bits are ignored
   reg [4:0] 			     mcr;
   reg [7:0] 			     lcr;
   reg [7:0] 			     msr;
   reg [15:0] 			     dl;  // 32-bit divisor latch
   reg [7:0] 			     scratch; // UART scratch register
   reg 				     start_dlc; // activate dlc on writing to UART_DL1
   reg 				     lsr_mask_d; // delay for lsr_mask condition
   reg 				     msi_reset; // reset MSR 4 lower bits indicator
   //reg 										threi_clear; // THRE interrupt clear flag
   reg [15:0] 			     dlc;  // 32-bit divisor latch counter
   reg 				     int_o;

   reg [3:0] 			     trigger_level; // trigger level of the receiver FIFO
   reg 				     rx_reset;
   reg 				     tx_reset;

   wire 			     dlab;			   // divisor latch access bit
   wire 			     cts_pad_i, dsr_pad_i, ri_pad_i, dcd_pad_i; // modem status bits
   wire 			     loopback;		   // loopback bit (MCR bit 4)
   wire 			     cts, dsr, ri, dcd;	   // effective signals
   wire 			     cts_c, dsr_c, ri_c, dcd_c; // Complement effective signals (considering loopback)
   wire 			     rts_pad_o, dtr_pad_o;		   // modem control outputs

   // LSR bits wires and regs
   wire [7:0] 			     lsr;
   wire 			     lsr0, lsr1, lsr2, lsr3, lsr4, lsr5, lsr6, lsr7;
   reg 				     lsr0r, lsr1r, lsr2r, lsr3r, lsr4r, lsr5r, lsr6r, lsr7r;
   wire 			     lsr_mask; // lsr_mask

   //
   // ASSINGS
   //

   assign 									lsr[7:0] = { lsr7r, lsr6r, lsr5r, lsr4r, lsr3r, lsr2r, lsr1r, lsr0r };

   assign 									{cts_pad_i, dsr_pad_i, ri_pad_i, dcd_pad_i} = modem_inputs;
   assign 									{cts, dsr, ri, dcd} = ~{cts_pad_i,dsr_pad_i,ri_pad_i,dcd_pad_i};

   assign                  {cts_c, dsr_c, ri_c, dcd_c} = loopback ? {mcr[`UART_MC_RTS],mcr[`UART_MC_DTR],mcr[`UART_MC_OUT1],mcr[`UART_MC_OUT2]}
   : {cts_pad_i,dsr_pad_i,ri_pad_i,dcd_pad_i};

   assign 									dlab = lcr[`UART_LC_DL];
   assign 									loopback = mcr[4];

   // assign modem outputs
   assign 									rts_pad_o = mcr[`UART_MC_RTS];
   assign 									dtr_pad_o = mcr[`UART_MC_DTR];

   // Interrupt signals
   wire 			     rls_int;  // receiver line status interrupt
   wire 			     rda_int;  // receiver data available interrupt
   wire 			     ti_int;   // timeout indicator interrupt
   wire 			     thre_int; // transmitter holding register empty interrupt
   wire 			     ms_int;   // modem status interrupt

   // FIFO signals
   reg 				     tf_push;
   reg 				     rf_pop;
   wire [`UART_FIFO_REC_WIDTH-1:0]   rf_data_out;
   wire 			     rf_error_bit; // an error (parity or framing) is inside the fifo
   wire 			     rf_overrun;
   wire 			     rf_push_pulse;   
   wire [`UART_FIFO_COUNTER_W-1:0]   rf_count;
   wire [`UART_FIFO_COUNTER_W-1:0]   tf_count;
   wire [2:0] 			     tstate;
   wire [3:0] 			     rstate;
   wire [9:0] 			     counter_t;

   wire 			     thre_set_en; // THRE status is delayed one character time when a character is written to fifo.
   reg [7:0] 			     block_cnt;   // While counter counts, THRE status is blocked (delayed one character cycle)
   reg [7:0] 			     block_value; // One character length minus stop bit

   // Transmitter Instance
   wire 			     serial_out;

   uart_transmitter transmitter(clk, wb_rst_i, lcr, tf_push, wb_dat_i, enable, serial_out, tstate, tf_count, tx_reset, lsr_mask);

   // Synchronizing and sampling serial RX input
   uart_sync_flops    i_uart_sync_flops
     (
      .rst_i           (wb_rst_i),
      .clk_i           (clk),
      .stage1_rst_i    (1'b0),
      .stage1_clk_en_i (1'b1),
      .async_dat_i     (srx_pad_i),
      .sync_dat_o      (srx_pad)
      );
   defparam i_uart_sync_flops.width      = 1;
   defparam i_uart_sync_flops.init_value = 1'b1;

   // handle loopback
   wire 			     serial_in = loopback ? serial_out : srx_pad;
   assign stx_pad_o = loopback ? 1'b1 : serial_out;

   // Receiver Instance
   uart_receiver receiver(clk, wb_rst_i, lcr, rf_pop, serial_in, enable, 
			  counter_t, rf_count, rf_data_out, rf_error_bit, rf_overrun, rx_reset, lsr_mask, rstate, rf_push_pulse);


   // Asynchronous reading here because the outputs are sampled in uart_wb.v file 
   always @(dl or dlab or ier or iir or scratch
	    or lcr or lsr or msr or rf_data_out or wb_addr_i or wb_re_i)   // asynchrounous reading
     begin
	case (wb_addr_i)
	  `UART_REG_RB   : wb_dat_o = dlab ? dl[`UART_DL1] : rf_data_out[10:3];
	  `UART_REG_IE	: wb_dat_o = dlab ? dl[`UART_DL2] : {4'd0,ier};
	  `UART_REG_II	: wb_dat_o = {4'b1100,iir};
	  `UART_REG_LC	: wb_dat_o = lcr;
	  `UART_REG_LS	: wb_dat_o = lsr;
	  `UART_REG_MS	: wb_dat_o = msr;
	  `UART_REG_SR	: wb_dat_o = scratch;
	  default:  wb_dat_o = 8'b0; // ??
	endcase // case(wb_addr_i)
     end // always @ (dl or dlab or ier or iir or scratch...


   // rf_pop signal handling
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)
	  rf_pop <=  0; 
	else
	  if (rf_pop)	// restore the signal to 0 after one clock cycle
	    rf_pop <=  0;
	  else
	    if (wb_re_i && wb_addr_i == `UART_REG_RB && !dlab)
	      rf_pop <=  1; // advance read pointer
     end

   wire 	lsr_mask_condition;
   wire 	iir_read;
   wire 	msr_read;
   wire 	fifo_read;
   wire 	fifo_write;

   assign lsr_mask_condition = (wb_re_i && wb_addr_i == `UART_REG_LS && !dlab);
   assign iir_read = (wb_re_i && wb_addr_i == `UART_REG_II && !dlab);
   assign msr_read = (wb_re_i && wb_addr_i == `UART_REG_MS && !dlab);
   assign fifo_read = (wb_re_i && wb_addr_i == `UART_REG_RB && !dlab);
   assign fifo_write = (wb_we_i && wb_addr_i == `UART_REG_TR && !dlab);

   // lsr_mask_d delayed signal handling
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)
	  lsr_mask_d <=  0;
	else // reset bits in the Line Status Register
	  lsr_mask_d <=  lsr_mask_condition;
     end

   // lsr_mask is rise detected
   assign lsr_mask = lsr_mask_condition && ~lsr_mask_d;

   // msi_reset signal handling
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)
	  msi_reset <=  1;
	else
	  if (msi_reset)
	    msi_reset <=  0;
	  else
	    if (msr_read)
	      msi_reset <=  1; // reset bits in Modem Status Register
     end


   //
   //   WRITES AND RESETS   //
   //
   // Line Control Register
   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i)
       lcr <=  8'b00000011; // 8n1 setting
     else
       if (wb_we_i && wb_addr_i==`UART_REG_LC)
	 lcr <=  wb_dat_i;

   // Interrupt Enable Register or UART_DL2
   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i)
       begin
	  ier <=  4'b0000; // no interrupts after reset
`ifdef PRESCALER_PRESET_HARD
	  dl[`UART_DL2] <=  `PRESCALER_HIGH_PRESET;
`else 
	  dl[`UART_DL2] <=  8'b0;
`endif	  
       end
     else
       if (wb_we_i && wb_addr_i==`UART_REG_IE)
	 if (dlab)
	   begin
	      dl[`UART_DL2] <=
`ifdef PRESCALER_PRESET_HARD
		dl[`UART_DL2];
`else
	      wb_dat_i;
`endif	      
	   end
	 else
	   ier <=  wb_dat_i[3:0]; // ier uses only 4 lsb


   // FIFO Control Register and rx_reset, tx_reset signals
   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) begin
	fcr <=  2'b11; 
	rx_reset <=  0;
	tx_reset <=  0;
     end else
       if (wb_we_i && wb_addr_i==`UART_REG_FC) begin
	  fcr <=  wb_dat_i[7:6];
	  rx_reset <=  wb_dat_i[1];
	  tx_reset <=  wb_dat_i[2];
       end else begin
	  rx_reset <=  0;
	  tx_reset <=  0;
       end

   // Modem Control Register
   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i)
       mcr <=  5'b0; 
     else
       if (wb_we_i && wb_addr_i==`UART_REG_MC)
	 mcr <=  wb_dat_i[4:0];

   // Scratch register
   // Line Control Register
   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i)
       scratch <=  0; // 8n1 setting
     else
       if (wb_we_i && wb_addr_i==`UART_REG_SR)
	 scratch <=  wb_dat_i;

   // TX_FIFO or UART_DL1
   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i)
       begin
`ifdef PRESCALER_PRESET_HARD	  
	  dl[`UART_DL1]  <=  `PRESCALER_LOW_PRESET;
`else
	  dl[`UART_DL1]  <=  8'b0;
`endif	  
	  tf_push   <=  1'b0;
	  start_dlc <=  1'b0;
       end
     else
       if (wb_we_i && wb_addr_i==`UART_REG_TR)
	 if (dlab)
	   begin
`ifdef PRESCALER_PRESET_HARD
	      dl[`UART_DL1] <= dl[`UART_DL1];
`else	      
	      dl[`UART_DL1] <=  wb_dat_i;
`endif	      
	      start_dlc <=  1'b1; // enable DL counter
	      tf_push <=  1'b0;
	   end
	 else
	   begin
	      tf_push   <=  1'b1;
	      start_dlc <=  1'b0;
	   end // else: !if(dlab)
       else
	 begin
	    start_dlc <=  1'b0;
	    tf_push   <=  1'b0;
	 end // else: !if(dlab)

   // Receiver FIFO trigger level selection logic (asynchronous mux)
   always @(fcr)
     case (fcr[`UART_FC_TL])
       2'b00 : trigger_level = 1;
       2'b01 : trigger_level = 4;
       2'b10 : trigger_level = 8;
       2'b11 : trigger_level = 14;
     endcase // case(fcr[`UART_FC_TL])
   
   //
   //  STATUS REGISTERS  //
   //

   // Modem Status Register
   reg [3:0] delayed_modem_signals;
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)
	  begin
  	     msr <=  0;
	     delayed_modem_signals[3:0] <=  0;
	  end
	else begin
	   msr[`UART_MS_DDCD:`UART_MS_DCTS] <=  msi_reset ? 4'b0 :
						msr[`UART_MS_DDCD:`UART_MS_DCTS] | ({dcd, ri, dsr, cts} ^ delayed_modem_signals[3:0]);
	   msr[`UART_MS_CDCD:`UART_MS_CCTS] <=  {dcd_c, ri_c, dsr_c, cts_c};
	   delayed_modem_signals[3:0] <=  {dcd, ri, dsr, cts};
	end
     end


   // Line Status Register

   // activation conditions
   assign lsr0 = (rf_count==0 && rf_push_pulse);  // data in receiver fifo available set condition
   assign lsr1 = rf_overrun;     // Receiver overrun error
   assign lsr2 = rf_data_out[1]; // parity error bit
   assign lsr3 = rf_data_out[0]; // framing error bit
   assign lsr4 = rf_data_out[2]; // break error in the character
   assign lsr5 = (tf_count==5'b0 && thre_set_en);  // transmitter fifo is empty
   assign lsr6 = (tf_count==5'b0 && thre_set_en && (tstate == /*`S_IDLE */ 0)); // transmitter empty
   assign lsr7 = rf_error_bit | rf_overrun;

   // lsr bit0 (receiver data available)
   reg 	 lsr0_d;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr0_d <=  0;
     else lsr0_d <=  lsr0;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr0r <=  0;
     else lsr0r <=  (rf_count==1 && rf_pop && !rf_push_pulse || rx_reset) ? 0 : // deassert condition
		    lsr0r || (lsr0 && ~lsr0_d); // set on rise of lsr0 and keep asserted until deasserted 

   // lsr bit 1 (receiver overrun)
   reg 	 lsr1_d; // delayed

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr1_d <=  0;
     else lsr1_d <=  lsr1;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr1r <=  0;
     else	lsr1r <= 	lsr_mask ? 0 : lsr1r || (lsr1 && ~lsr1_d); // set on rise

   // lsr bit 2 (parity error)
   reg 	 lsr2_d; // delayed

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr2_d <=  0;
     else lsr2_d <=  lsr2;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr2r <=  0;
     else lsr2r <=  lsr_mask ? 0 : lsr2r || (lsr2 && ~lsr2_d); // set on rise

   // lsr bit 3 (framing error)
   reg 	 lsr3_d; // delayed

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr3_d <=  0;
     else lsr3_d <=  lsr3;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr3r <=  0;
     else lsr3r <=  lsr_mask ? 0 : lsr3r || (lsr3 && ~lsr3_d); // set on rise

   // lsr bit 4 (break indicator)
   reg 	 lsr4_d; // delayed

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr4_d <=  0;
     else lsr4_d <=  lsr4;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr4r <=  0;
     else lsr4r <=  lsr_mask ? 0 : lsr4r || (lsr4 && ~lsr4_d);

   // lsr bit 5 (transmitter fifo is empty)
   reg 	 lsr5_d;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr5_d <=  1;
     else lsr5_d <=  lsr5;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr5r <=  1;
     else lsr5r <=  (fifo_write) ? 0 :  lsr5r || (lsr5 && ~lsr5_d);

   // lsr bit 6 (transmitter empty indicator)
   reg 	 lsr6_d;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr6_d <=  1;
     else lsr6_d <=  lsr6;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr6r <=  1;
     else lsr6r <=  (fifo_write) ? 0 : lsr6r || (lsr6 && ~lsr6_d);

   // lsr bit 7 (error in fifo)
   reg 	 lsr7_d;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr7_d <=  0;
     else lsr7_d <=  lsr7;

   always @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) lsr7r <=  0;
     else lsr7r <=  lsr_mask ? 0 : lsr7r || (lsr7 && ~lsr7_d);

   // Frequency divider
   always @(posedge clk or posedge wb_rst_i) 
     begin
	if (wb_rst_i)
	  dlc <=  0;
	else
	  if (start_dlc | ~ (|dlc))
  	    dlc <=  dl - 1;               // preset counter
	  else
	    dlc <=  dlc - 1;              // decrement counter
     end

   // Enable signal generation logic
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)
	  enable <=  1'b0;
	else
	  if (|dl & ~(|dlc))     // dl>0 & dlc==0
	    enable <=  1'b1;
	  else
	    enable <=  1'b0;
     end

   // Delaying THRE status for one character cycle after a character is written to an empty fifo.
   always @(lcr)
     case (lcr[3:0])
       4'b0000                             : block_value =  95; // 6 bits
       4'b0100                             : block_value = 103; // 6.5 bits
       4'b0001, 4'b1000                    : block_value = 111; // 7 bits
       4'b1100                             : block_value = 119; // 7.5 bits
       4'b0010, 4'b0101, 4'b1001           : block_value = 127; // 8 bits
       4'b0011, 4'b0110, 4'b1010, 4'b1101  : block_value = 143; // 9 bits
       4'b0111, 4'b1011, 4'b1110           : block_value = 159; // 10 bits
       4'b1111                             : block_value = 175; // 11 bits
     endcase // case(lcr[3:0])

   // Counting time of one character minus stop bit
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)
	  block_cnt <=  8'd0;
	else
	  if(lsr5r & fifo_write)  // THRE bit set & write to fifo occured
	    block_cnt <=  block_value;
	  else
	    if (enable & block_cnt != 8'b0)  // only work on enable times
	      block_cnt <=  block_cnt - 1;  // decrement break counter
     end // always of break condition detection

   // Generating THRE status enable signal
   assign thre_set_en = ~(|block_cnt);


   //
   //	INTERRUPT LOGIC
   //

   assign rls_int  = ier[`UART_IE_RLS] && (lsr[`UART_LS_OE] || lsr[`UART_LS_PE] || lsr[`UART_LS_FE] || lsr[`UART_LS_BI]);
   assign rda_int  = ier[`UART_IE_RDA] && (rf_count >= {1'b0,trigger_level});
   assign thre_int = ier[`UART_IE_THRE] && lsr[`UART_LS_TFE];
   assign ms_int   = ier[`UART_IE_MS] && (| msr[3:0]);
   assign ti_int   = ier[`UART_IE_RDA] && (counter_t == 10'b0) && (|rf_count);

   reg 	 rls_int_d;
   reg 	 thre_int_d;
   reg 	 ms_int_d;
   reg 	 ti_int_d;
   reg 	 rda_int_d;

   // delay lines
   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) rls_int_d <=  0;
     else rls_int_d <=  rls_int;

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) rda_int_d <=  0;
     else rda_int_d <=  rda_int;

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) thre_int_d <=  0;
     else thre_int_d <=  thre_int;

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) ms_int_d <=  0;
     else ms_int_d <=  ms_int;

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) ti_int_d <=  0;
     else ti_int_d <=  ti_int;

   // rise detection signals

   wire  rls_int_rise;
   wire  thre_int_rise;
   wire  ms_int_rise;
   wire  ti_int_rise;
   wire  rda_int_rise;

   assign rda_int_rise    = rda_int & ~rda_int_d;
   assign rls_int_rise 	  = rls_int & ~rls_int_d;
   assign thre_int_rise   = thre_int & ~thre_int_d;
   assign ms_int_rise 	  = ms_int & ~ms_int_d;
   assign ti_int_rise 	  = ti_int & ~ti_int_d;

   // interrupt pending flags
   reg 	 rls_int_pnd;
   reg 	 rda_int_pnd;
   reg 	 thre_int_pnd;
   reg 	 ms_int_pnd;
   reg 	 ti_int_pnd;

   // interrupt pending flags assignments
   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) rls_int_pnd <=  0; 
     else 
       rls_int_pnd <=  lsr_mask ? 0 :  						// reset condition
		       rls_int_rise ? 1 :						// latch condition
		       rls_int_pnd && ier[`UART_IE_RLS];	// default operation: remove if masked

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) rda_int_pnd <=  0; 
     else 
       rda_int_pnd <=  ((rf_count == {1'b0,trigger_level}) && fifo_read) ? 0 :  	// reset condition
		       rda_int_rise ? 1 :						// latch condition
		       rda_int_pnd && ier[`UART_IE_RDA];	// default operation: remove if masked

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) thre_int_pnd <=  0; 
     else 
       thre_int_pnd <=  fifo_write || (iir_read & ~iir[`UART_II_IP] & iir[`UART_II_II] == `UART_II_THRE)? 0 : 
			thre_int_rise ? 1 :
			thre_int_pnd && ier[`UART_IE_THRE];

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) ms_int_pnd <=  0; 
     else 
       ms_int_pnd <=  msr_read ? 0 : 
		      ms_int_rise ? 1 :
		      ms_int_pnd && ier[`UART_IE_MS];

   always  @(posedge clk or posedge wb_rst_i)
     if (wb_rst_i) ti_int_pnd <=  0; 
     else 
       ti_int_pnd <=  fifo_read ? 0 : 
		      ti_int_rise ? 1 :
		      ti_int_pnd && ier[`UART_IE_RDA];
   // end of pending flags

   // INT_O logic
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)	
	  int_o <=  1'b0;
	else
	  int_o <=  
		    rls_int_pnd		?	~lsr_mask					:
		    rda_int_pnd		? 1								:
		    ti_int_pnd		? ~fifo_read					:
		    thre_int_pnd	? !(fifo_write & iir_read) :
		    ms_int_pnd		? ~msr_read						:
		    0;	// if no interrupt are pending
     end


   // Interrupt Identification register
   always @(posedge clk or posedge wb_rst_i)
     begin
	if (wb_rst_i)
	  iir <=  1;
	else
	  if (rls_int_pnd)  // interrupt is pending
	    begin
	       iir[`UART_II_II] <=  `UART_II_RLS;	// set identification register to correct value
	       iir[`UART_II_IP] <=  1'b0;		// and clear the IIR bit 0 (interrupt pending)
	    end else // the sequence of conditions determines priority of interrupt identification
	      if (rda_int)
		begin
		   iir[`UART_II_II] <=  `UART_II_RDA;
		   iir[`UART_II_IP] <=  1'b0;
		end
	      else if (ti_int_pnd)
		begin
		   iir[`UART_II_II] <=  `UART_II_TI;
		   iir[`UART_II_IP] <=  1'b0;
		end
	      else if (thre_int_pnd)
		begin
		   iir[`UART_II_II] <=  `UART_II_THRE;
		   iir[`UART_II_IP] <=  1'b0;
		end
	      else if (ms_int_pnd)
		begin
		   iir[`UART_II_II] <=  `UART_II_MS;
		   iir[`UART_II_IP] <=  1'b0;
		end else	// no interrupt is pending
		  begin
		     iir[`UART_II_II] <=  0;
		     iir[`UART_II_IP] <=  1'b1;
		  end
     end

endmodule
